Generally, a cooling cycle used in refrigerators or air conditioners is designed to reduce the temperature of the ambient air by compressing a coolant gas using a compressor, liquefying the compressed coolant gas using a condenser, reducing the pressure of the liquified coolant through an expansion valve, and evaporating the coolant in an evaporator. Particularly, in recent years, the cooling temperature has been automatically controlled by automatically controlling the RPMs of a compressor according to a target temperature using a frequency converting process such as an inverter control method.
Japanese unexamined patent No. S60-196477 discloses an electronic expansion valve which can actively adjust an amount of coolant according to a variation of the RPMs of a compressor through an inverter controller. The expansion valve will be described hereinbelow with reference to FIG. 1.
As shown in the drawing, the electronic expansion valve comprises a valve part V and a stepping motor part M.
The valve part V comprises a valve housing 1 provided at its side with a coolant intake hole 1a on which a coolant tube A connected to a condenser (not shown) is coupled and at its bottom or distal end with a coolant exhaust hole 1b on which a coolant tube B connected to an evaporator(not shown) is coupled. A valve seat 2a is disposed on a partition wall 2 between the intake and exhaust holes 1a and 1b. A needle valve 3 is disposed to contact or move away from the valve seat 2a. That is, the needle valve 3 is slidable in an axial direction and supported within a male screw tube 4 screw-coupled on the valve housing 1.
A sealed case 6 defining the stepping motor part M is disposed on a top or proximal end of the valve housing 1. A stator 8 with a coil 7 is disposed around the case 6 and a rotor 9 is disposed inside the case 6 while being rotatably supported by the mail screw tube 4.
The rotor 9 comprises a permanent magnet 11 embedded around a supporting case 10. A female screw tube 12 is fixed on a lower-inner wall of the supporting case 10 and screw-coupled to the male screw tube 4 so that the rotor 9 is rotatable while being slidable in an axial direction.
A connecting wall 10b is defined at a middle-inner portion of the supporting case 10 and is provided with a penetrating hole 10c in which a smaller diameter portion 3a of the needle valve 3 is fixedly inserted. A coil spring 14 is disposed around the smaller diameter portion 3a of the needle valve 3, while being abutted to the connecting wall 10b.
A center rod 15 is fixed between the rotor 9 and the case 6, and a spiral guide ring 16 is disposed around the center rod 15. A slider 17 is coupled to the spiral guide ring 16 such that it can rotate while moving in a vertical direction along the spiral guide ring 16. An outer end of the slider 17 is hooked on a support rod 18 located on the rotor 9.
In operation, when electric power is applied to the coil 7 of the stator 8, the rotor 9 rotates to ascend and descend the needle valve 3 and the slider 17 along the guide ring 16. At this point, the rotation of the rotor 9 and the movement of the needle valve 3 are stopped at positions where the slider 17 contacts upper and lower ends 16a and 16b of the guide ring 16. That is, by moving the needle valve 3 in the vertical direction, an amount of coolant passing through the valve housing 1 can be properly adjusted, thereby adjusting the pressure.
However, in the above described electronic expansion valve, since the rotor 9 is disposed inside the case 6 and the stator 8 is disposed outside the case 6, an air gap between the coil 7 of the stator 8 and the permanent magnet 11 of the rotor 9 is increased. Accordingly, to precisely generate the torque required for the expansion valve, a relatively large or expensive magnet, e.g. a rare-earth magnet, should be used, increasing the size and manufacturing costs of the expansion valve.
Furthermore, to restrict the rotation range of the rotor 9 and a vertical movement range of the needle valve 3, since the center rod 15, the guide ring 16, the slider 17 and the support rod 18 should be disposed on the top of the rotor 9 disposed inside the case 6, the longitudinal length of the expansion valve is increased.
An initial position of the needle valve 3 is set by screw-coupling the male screw tube 4 to the valve housing 1, then the female screw tube 12 of the rotor 9 to the male screw tube 4. Generally, the initial position of the needle valve 3 is set at a position where the needle valve 3 closely contacts the valve seat 2a to completely close the fluid path. In this initial position, the slider 17 is located at the lower end 16b of the guide ring 16, the slider 17 should then be secured on the support rod 18 located on the rotor 9. However, since there is a limitation in precisely moving the screw, it is very difficult to accurately set the initial position of the needle valve 3 by screw-coupling the valve housing 1 to the male screw tube 4 and the female screw tube 12 to the male screw tube 4. Furthermore, when assembling the expansion valve, there is the possibility that the slider 17 is not exactly secured on the support rod 18 even after the initial position of the needle valve 3 is set.
If the expansion valve is operated in a state where the slider 17 is not exactly secured on the support rod 18, although the needle valve 3 starts its vertical movement from its initial position by the rotation of the rotor 9, the slider 17 stays at the lower end 16b of the guide ring 16 until it contacts the support rod 18. That is, the distance of the vertical movement of the needle valve 3 is defined by the slider 17 which moves between the upper and lower ends 16a and 16b of the guide ring 16. At this point, since the slider 17 starts its vertical movement after the support rod 18 contacts the slider 17, a target distance of the vertical movement of the needle valve 3 may not be obtained, making it difficult to precisely adjust an amount of coolant to be exhausted.